SUPRAS in High-Tc Update Newsletters
High-Tc Update
newsletters were started in April 1987 as a short-term
solution for a crisis situation: the need to communicate frequent--almost
daily--breakthroughs in high-Tc superconductivity. Over the years,
it has become a successful model for how research results can be very
effectively communicated.
High-Tc Update newsletters include a preprint review section
called "Nota Bene" which summarizes the most significant recent
results in the field.
SUPRAS appears frequently in this review
section; the comments are listed below.
According to a preprint by C. Hannay et al. (Liege), different
thermal cycles have been used to synthesize DyBa2Cu3O7-x
superconducting ceramics in situ under a 0.6 T applied magnetic
field at low and high temperature. Much grain texturing is found,
and new features (pyramid-like growth from terrace stacking) are
illustrated. The resulting grain alignments, however, did not
result in improved levels of the critical-current density.
[High-Tc Update 15 april 1992]
In a step toward the development of a model for the atom-by-atom
growth of magnetically textured high-Tc superconductors, M. Ausloos
(Liege) et al. have introduced a magnetic Eden model, which is the
Eden model with an extra degree of freedom. The authors use the
new model to simulate the growth of clusters in two dimensions and
to discuss the transitions between compact and lacunar clusters.
[High-Tc Update 15 november 1993]
The effect of sulfur substitution in YBCO, which results in an
amorphous YBCO-S layer between YBCO grains, has been studied by R.
Cloots (Liege) et al. using mu^+ spin rotation/relaxation
techniques in zero and applied magnetic fields. The authors find
that the new phase acts as a magnetic impurity.
[High-Tc Update 01 january 1994]
A brief review article on oxygen diffusion in YBa2Cu3O7-d has been
prepared by M. Ausloos (Liege) and A. Pekalski (Wroclaw). The
authors discuss the results and interpretations of various
experiments, and they note that it is essential to account for
oxygen-vacancy next-nearest-neighbor (NNN) hopping in the basal
plane to reconcile theory and experiment. The authors also discuss
a newly found V-Delta transition in the tracer diffusion
coefficient as a function of oxygen concentration at low
temperature (31 references).
[High-Tc Update 01 october 1994]
A preprint by R. Cloots et al. (Liege) discusses the crystal-growth
mechanism of 123 superconducting ceramics using scanning electron
micrographs of DyBa2Cu3O7-d synthesized by peritectic recombination
under a magnetic field of 0.6 T. The authors introduce a growth
model, do computer simulations, and find that they can explain all
the microstructural data reported for such textured materials.
[High-Tc Update 15 november 1994]
The Liege Ph.D. thesis of H. Bougrine describes the method,
experimental set-up, and related measurements with a simultaneous
data acquisition technique on the thermal conductivity and
thermoelectric power of the high-temperature superconductors
YBa2Cu3O7-d (Y-123, including samples doped with Fe),
Bi2Sr2CaCu2O8+d (Bi-2212), and (Bi,Pb)2Sr2Ca2Cu3O10 [(Bi,Pb)-2223].
The author found that in Y-123 the magnetic-field-dependent
thermal conductivity has a complicated behavior with several
crossovers between various mechanisms due to free and pinned
vortices and the interplay of various characteristic lengths (grain
size, coherence length, and penetration depth).
[High-Tc Update 15 november 1994]
Measurements of the magnetic-field dependence of the thermal
conductivity of an untwinned YBa2Cu3O7-d single crystal and
YBa2Cu3O7-d and YBa2(Cu0.95Fe0.05)3O7-d ceramics are reported by M.
Ausloos and M. Houssa (Liege). The authors find that the results
can be understood by assuming that the main carriers of heat are
electrons scattered by the vortex cores.
[High-Tc Update 15 march 1995]
A kinetic growth model describing the details of melt-textured
growth of R-123 compounds is discussed in a preprint by N.
Vandewalle et al. (Liege). Results of computer modeling also are
presented.
[High-Tc Update 15 march 1995]
The contributions to the electronic thermal conductivity of two
types of electron scatterers in type-II superconductors, free
(unpinned) vortices and pinned (via extended defects) vortices, are
discussed by S. Sergeenkov and M. Ausloos (Liege). The theoretical
predictions are found to be in reasonable agreement with
experimental data on the thermal conductivity of twinned and
tweeded high-Tc superconductors in high magnetic fields.
[High-Tc Update 01 june 1995]
The transport properties of Na-doped, Ba-deficient YBa2Cu3O7-d thin
films have been studied by M. Pekala (Warsaw and Liege) et al. The
films had small grains with c axes oriented perpendicular to the
MgO substrate. The electrical resistivity and thermopower were
studied as a function of temperature and magnetic field. The
results are explained in terms of weak links.
[High-Tc Update 01 july 1995]
As reported by M. Ausloos (U. Liege) et al., the resistive
transition R(T) between the Kosterlitz-Thouless (K-T) temperature
and the Ginzburg-Landau temperature was studied in spray-dried bulk
Bi2Sr2CaCu2O8+d superconductors for magnetic fields between 0 and
0.3 Tesla. The results are in quantitative agreement with the two-
dimensional (2D) K-T theory. This implies that the R(T) behavior
can be described within a picture of a highly 2D behavior of
macroscopic fluctuations of vortex-antivortex depairing type. The
role of defects or inhomogeneities on R(T) is discussed.
[High-Tc Update 15 july 1995]
Monte-Carlo simulations of the kinetics of growth of
Y1Ba2Cu3O7-d grains have been carried out by Cloots et al.
(Liege). Using simple kinetic and geometric arguments, they
develop a computer model that simulates growth in the a-b plane.
Both chemical-bonding anisotropy and the presence of
Y2Ba1Cu1O5-d precipitates are considered. The anisotropy of the
211 phase trapping was found to be an important ingredient in
the development of some of the patterns observed in the growth.
[High-Tc Update 15 august 1995]
The microstructure of magnetically melt-textured DyBa2Cu3O7-d
samples as observed by polarized-light metallography is reported by
P. Diko (Kosice) et al. The authors discuss phase dimensions,
morphology, orientation, nature, and distribution, and they
characterize the grains, twin-structure pattern, and grain
boundaries.
[High-Tc Update 15 september 1995]
The temperature dependence of the thermal conductivity of high-Tc
superconductors with a superconducting energy gap of either s-wave or
d_[x^2-y^2] symmetry has been considered theoretically by M. Houssa
and M. Ausloos (Liege). The authors analyzed existing experimental
results on the low-temperature behavior of the thermal conductivity
of La2-xSrxCuO4, YBa2Cu3O7-d, Bi2Sr2CaCu2O8+d, and Tl2Ba2Ca2Cu3O10
compounds. The authors found that this behavior is inconsistent with
an isotropic s-wave gap parameter but that it can be explained by
assuming a gap of d_[x^2-y^2] symmetry.
[High-Tc Update 01 october 1995]
A model for the two-dimensional growth of a solid interface through a
liquid in the presence of particles that are pushed by the advancing
front is proposed in a preprint by N. Vandewalle and M. Ausloos
(Liege). A transition between growing clusters and frozen ones occurs
for a critical particle fraction x_c = 0.560+-0.005. At x_c, both
percolating clusters and internal patterns are fractal with the same
fractal dimension D = 1.87+-0.03, which is close to the classical
percolation exponent 91/48. The authors discuss this model in
connection with the growth of YBa2Cu3O7-d (123) in the presence of
Y2BaCuO5 (211) particles.
[High-Tc Update 15 november 1995]
A preprint by M. Ausloos (Liege) et al. describes a kinetics mechanism that
gives rise to dendritic morphology in YBa2Cu3O7-d crystals. The authors
present results of Monte Carlo simulations that exhibit dendritic growth.
[High-Tc Update 01 january 1996]
The temperature dependence of the in-plane electronic contribution to
the thermal conductivity of a d-wave superconductor has been calculated by
M. Houssa and M. Ausloos (Liege) using a variational method.
To account for the layered structure of high-Tc compounds,
the authors consider an anisotropic Lawrence-Doniach energy spectrum
for the electrons. The low-temperature behavior of the thermal
conductivity of most high-Tc superconductors and the peak structure
observed below Tc are quantitatively reproduced by this model.
These results suggest that the order parameter of the high-Tc superconductors
has d_[x^2-y^2] symmetry, and support the hypothesis that the peak observed
in the thermal conductivity of these materials has an electronic origin.
[High-Tc Update 01 january 1996]
The mixed-state resistive tail of sintered YBa2Cu3O7-d has been studied by
V. V. Gridin (Witwatersrand) et al. using a Corbino-disk sample geometry.
When the transport current passes radially from the rim of the sample to
its center, the authors argue that the voltage signal consists of two
components: one due to quasiparticles and the other due to vortex motion.
The authors claim that the quasiparticle contribution is comparable with
that from vortex motion.
[High-Tc Update 15 january 1996]
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Nicolas Vandewalle
E-mail: vandewal@gw.unipc.ulg.ac.be
last update: March 24, 1996